A distribution of inherent stress wherein the resultant load stress in the most highly loaded cross-section of the finished spring never reaches the compression or tension flow threshold at any point in the cross-section and a critical tension of 200 N/mm.sup.2 is never exceeded in the marginal area near the surface.
There are two major methods of intentionally affecting inherent stresses in springs at the state of the art, specifically setting and ball bombardment. They modify the inherent-stress distribution primarily in a specific cross-sectional areas associated with the particular method. Setting, or plasticizing, means the initial subjection to load of a spring once shaped and treated whereby the material's flow threshold is exceeded in some areas of the spring's cross-section in accordance with the type of spring and stress.
The ratio between the plasticized cross-sectional area and the overall cross-section is called the degree P of plasticization. It dictates in conjunction with the material-specific flow threshold the level and distribution of the inherent stresses induced by the setting operation.
Locally demarcated alteration of the flow threshold provoked by the prescription for a specific gradient of strength or temperature in the spring's cross-section during the setting operation can accordingly be used to modify both the level and distribution of inherent stress in the sense of optimization.
Whereas the inherent stresses are subjected to directional modifications over the total cross-section depending on the type of stress, monaxial during bending or biaxial during torsion, and on the direction of stress, specifically tension or compression, during setting, the area affected by ball bombardment is essentially restricted to the marginal areas near the surface, whereby an orthogonal distribution of stress is produced in the outer layer independent of the type of spring or stress.
U.S. Pat. No. 2,608,752 describes, with a single-leaf spring as an example, a method whereby the tension side of the spring is stressed (60-100% R.sub.e) and bombarded with balls. The method has been introduced into practice as "stressed bombardment", and many versions are now being employed worldwide for single-leaf and parabolic springs.
Machinery and devices for bombarding leaf springs and various versions of the method are described in U.S. Pat. No. 3,094,768 and GB Patent 959 801.
An alternative to stressed bombardment that achieves a similar effect without the drawback of relatively major distortion is the bombardment method described in U.S. Pat. No. 3,205,556, whereby leaf springs are bombarded with balls while unloaded (without being tensioned that is) but at high temperatures (150.degree.-350.degree. C.). This "heated bombardment" has not as yet been successful in practice. One reason is that recent developments make it possible to compensate for the more severe distortions that accompany stressed bombardment simply by modifying the bending tool. Another is that heated bombardment does not as a rule extend the life of the spring to that extent that stressed bombardment does.
Common to all the publications hereintofore cited is that the fields of application they describe by way of example strictly relate to leaf springs that are essentially stressed monaxially.
Valid predictions as to life extension cannot, especially considering the sequence of individual operations in conjunction with optimization of the inherent-stress distribution, be applied to torsionally stressed springs without taking the essentially biaxial stress into consideration.
Setting followed by bombardment of helical compression springs, however, is known from the 1987 Hoesch Hohenlimburg AG publication "Warmgeformte Federn".